The long-term aim of this project is to understand the molecular basis of the modification of enzyme activities and the regulation of enzyme biosynthesis as it is exemplified by the proteins involved in the biosynthesis of the aromatic amino acids phenylalanine, tyrosine and tryptophan in Escherichia coli K 12. DAHP (3-deoxy-D-arabino-heptulosonic acid 7-phosphate)-synthetase (E.C. 4.1.2.15) is the very first enzyme of the pathway which finally leads to the synthesis of those three amino acids. The enzyme appears in three isoenzymic forms, one of which is feedback-inhibited by tyrosine, one by phenylalanine, and the third one by tryptophan. My first objective is to determine structural differences between two of the isoenzymes that we obtained as homogeneous preparations. There is evidence that one of the isoenzymes is modified in response to different growth conditions, that are unrelated to feedback inhibition. The second objective is to find out the nature of this modification. Finally, I would like to study regulatory mutants, effecting the tyrosine sensitive isoenzyme, that were recently isolated in my laboratory and had not been described before.